Who might that be?

Short list right now is Whitmer, Moore, Pritzker, Buttigieg, Shapiro, Booker, Newsom, Beshear, Klobuchar. Maybe Polis, maybe Booker. Probably not Emmanuel, though he's sniffing around it. Probably at least one non-politician gets enough traction to be talked about, to be this cycle's Andrew Yang (whether that's Smith or someone else who's hitting the zeitgeist in two years). At least one other Rep that manages to catch fundraising juice.

Whoever it is, some non-trivial chunk of the Democratic party will be disappointed that it's them and not one of the other candidates. And for the Democrats to win, that non-trivial chunk will need to manage that disappointment and strive with enthusiasm to get the candidate they are disappointed with elected - being mindful of the difference.

The GOP was able to do this with their sizable pro-life faction. They were deeply disappointed with Trump last cycle. He basically stripped their hard-won language on abortion out of the platform, signaled he would appoint a pro-life person to head HHS, all-but-turned the party into a pro-choice party for blue states, and indicated he would not under any circumstances stand up for abortion restrictions during the campaign. But they swallowed that disappointment, didn't stray with alternative candidates, and didn't punish him to try to force him to move on their issues. They let him win the election.

BTW, they have to be enormously disappointed with the early days of the Trump Administration. He has blown every norm out of the water, smashed every Overton Window he can find, pulled every single lever of power no matter how outlandish....but has done virtually nothing to restrict abortion. He's been blasting in every direction one every issue except that one.